The present invention relates to a throttle value control apparatus for an internal combustion engine which controls the opening of a throttle valve through the action of an electronically controlled actuator.
In recent years, many throttle valve control apparatuses having electronically controlled actuators have been developed for the purpose of improving the driving feeling and running performance of a vehicle. A conventional mechanical linkage connected between an accelerator pedal and a throttle valve through a wire cable is replaced by a system having an electronically controlled actuator interposed between the accelerator pedal and the throttle value for controlling the throttle valve based on the engine operating conditions and the running conditions of a vehicle in addition to the operational quantity of the accelerator pedal imparted by the driver.
With this type of throttle value control apparatus equipped with an electronically controlled actuator, however, the throttle valve is controlled to open and close by the actuator in the form of an electric motor which is in turn controlled by control commands from a vehicle control unit including a calculation and control section which operates to successively calculate an optimal degree of opening for the throttle valve in response to signals representative of the engine operating conditions, the running conditions of the vehicle and the like. As a result, in order to ensure safety in driving, it is necessary to provide a fail-safe means for preventing run-away of the vehicle during the time when the electronically controlled actuator fails. A typical example of such a fail-safe means is known from Japanese Patent Publication No. 61-54933 in which a throttle valve, which is controllably driven by an electrically operated actuator, and another safety throttle valve operatively connected with an accelerator pedal are disposed in the intake pipe of the engine in a serial relation with each other. Also, another typical example is shown in Japanese Patent Laid-Open No. 61-60331 in which a first throttle valve, which is operatively connected with an accelerator pedal, and a second throttle valve, which is controllably driven by an electric motor, are disposed in the intake pipe of the engine in a serial relation with each other so as to control the slip rate of the road wheels of a vehicle.
However, the conventional throttle valve control apparatuses as constructed above have the following disadvantages. Namely, with the above conventional throttle valve control apparatuses in which the two throttle valves one being operatively connected with the accelerator pedal and the other being controlled by the electically controlled actuator, are serially disposed in the intake pipe, it is possible to prevent run-away of the vehicle by controlling the one of the throttle valves operatively connected with the accelerator pedal upon failure of the electronically operated actuator, but the serial disposition of the two throttle valves necessarily results in an enlargement of the entire apparatus, creating difficulty in installing the apparatus inside an engine compartment which is ordinarily limited in space. Further, there is another problem that it is impossible to make the vehicle run in an automatic manner without any accelerator pedal operation by the driver as in the case of cruise control in which the vehicle is automatically run at a present constant speed.